1) Field of the Invention
This invention relates to screens for r.f. magnetic flux.
2) Description of Related Art
Screening or shielding components from r.f. magnetic fields, and conversely preventing them from radiating r.f. energy, is generally done by encasing the components in a conductor. The conductor must be fully sealed, and have a thickness that greatly exceeds the skin depth at the relevant frequency. R.f. coils (for example, in r.f. communication systems, particularly at high powers and low frequencies) are generally shielded by enclosing them in a copper or aluminium can. The shield must completely enclose the component to be shielded, and all joints should be lapped or soldered. This approach tends to be costly, and often carries with it a weight penalty. In MRI (magnetic resonance imaging) systems, the entire apparatus is housed in a copper-clad room, with special precautions being taken to ensure that the door is fully shielded when closed, by providing copper contacts between the door and the frame, and a copper brush to contact the floor.
An alternative method of shielding is to enclose the region to be protected in a magnetic shell such as ferrite or mu-metal. This approach is clearly impossible for MRI and related applications, where there are strong DC or low frequency magnetic fields: the magnetic material would simply be attracted to the DC magnet with such force as to destroy the original magnetic shell.
Ferrites are also used as filters in many electronic systems that operate at high frequencies. For example, there are ferrite filters in PCs and monitors, to prevent the radiation of the clock frequency along interconnecting cables. In the MRI system, where ferrites are unacceptable, all the cabling that connects the machine to the outside world must have filters to suppress the r.f. These filters have to be constructed from either active or passive electronic circuits.
Microstructured materials have been proposed (IEEE Transactions on Microwave Theory and Techniques, 1999, 47, 2075-2084, Magnetism from Conductors and Enhanced Non-Linear Phenomena by J B Pendry, A J Holden, D J Robbins and W J Stewart, and International Patent Application WO 00/41270), in which the magnetic permeability depends on the frequency of oscillation of the magnetic field i.e. depends on the r.f. frequency.
Such materials comprise a structure with magnetic properties comprising an array of capacitive elements, the array exhibiting a predetermined magnetic permeability in response to incident electromagnetic radiation lying within a predetermined frequency band, wherein each element includes a low resistance conducting path and is such that a magnetic component of the electromagnetic radiation lying within the predetermined frequency band induces an electrical current to flow around said path and through said associated element, and wherein the size of the elements and their spacing apart are selected such as to provide a predetermined permeability in response to said received electromagnetic radiation. At least one dimension of each capacitive element is preferably less than the wavelength of the electromagnetic radiation. The capacitive elements may be in the form of conductive sheets wound as a spiral, or a plurality of stacked planar sections each of which is electrically isolated from each other and is in the form of a spiral. The further preferred features of the materials are disclosed and claimed in International Patent Application WO 00/41270 and United Kingdom Patent Application No. 0005356.1, the contents of each of which are incorporated herein by reference.
Typically such materials consist of an array of capacitive elements at least one of whose dimensions is much smaller than the wavelength at which the desired permeability is exhibited, in which resonant interaction between the inductance of the structure and the capacitive elements causes electromagnetic energy to be shared between the magnetic fields and the electrostatic fields within the capacitive structure.
The invention provides a screen for r.f. magnetic flux, comprising an array of capacitive elements, the array exhibiting a predetermined magnetic permeability in response to incident electromagnetic radiation lying within a predetermined frequency band, wherein each capacitive element includes a low resistance conducting path and is such that a magnetic component of the electromagnetic radiation lying within the predetermined frequency band induces an electrical current to flow around said path and through said associated element, wherein the spacing of the elements is less than the wavelength of the radiation within the predetermined frequency band, and wherein the size of the elements and their spacing apart is selected such as to provide a magnetic permeability having a zero or negative real part to electromagnetic radiation in the predetermined frequency band.
The microstructured magnetic materials can be tuned to a required frequency, and constructed from readily-available materials which are both cheap and light. The material is non-magnetic in steady magnetic fields. The material may be attached to cables to prevent them radiating, reducing or obviating the need for filters, and may be draped around regions it is desired to shield.
Advantageously, the predetermined frequency band is within the HF band extending from 3 MHz to 30 MHz. Preferably, the predetermined frequency band is within the range extending from approximately 19 MHz to approximately 39 MHz.
The screen may form a planar structure. The capacitive elements may be rolls of conducting material arranged with their axes normal to the local surface of the planar structure. Alternatively, the planar structure is formed by a plurality of layers, each layer bearing an array of planar rings or spirals.
The spacing of the elements may be less than one half, preferably less than one fifth, of the wavelength of the radiation within the predetermined frequency band. Further advantages may flow from making the element spacing less than a tenth, or less than one hundredth of the wavelength of the radiation within the predetermined frequency band.
The invention also provides a method of designing a screen for magnetic r.f. flux, which comprises selecting the dimensions of capacitive elements arranged in an array, the array exhibiting a predetermined magnetic permeability in response to incident electromagnetic radiation lying within a predetermined frequency band, in which capacitive elements currents can be induced by the radio frequency electromagnetic radiation, and selecting the spacing apart of the capacitive elements to be less than the wavelength of the radiation within the predetermined frequency band, and the size and spacing of the elements in order to provide a magnetic permeability for the screen in response to the electromagnetic radiation within the predetermined frequency band which has a zero or negative real part.